Add instructions to unvendor the runtime

README.md

@@ -10,8 +10,14 @@ In your Cython+ projects, you can add this repository as a [git submodule](https
 at the root of your project:
 ```sh
 git submodule add https://lab.nexedi.com/cython-plus/runtime.git
+git commit -m "Register the runtime as a git submodule"
 ```
 
-This way, you can version your code with a specific version of the runtime and update it when needed.
-
+If you were previously vendoring the runtime, you need to remove the associated directory from the repository cache first:
+```sh
+git rm --cached -r runtime/
+git submodule add https://lab.nexedi.com/cython-plus/runtime.git
+git commit -m "Unvendor the runtime and instead register it as a git submodule"
+```
 
+This way, you can version your code with a specific version of the runtime and update it when needed.